Age-old Mystery Of Missing Chemicals From Earth’s Mantle May Be Solved

Observations about the early formation of Earth may answer an age-old question about why the planet’s mantle is missing some of the matter that should be present, according to UBC geophysicist John Hernlund.

Earth is made from chondrite, very primitive rocks of meteorites that date from the earliest time of the solar system before the Earth was formed. However, scientists have been puzzled why the composition of Earth’s mantle and core differed from that of chondrite.

Hernlund’s findings suggest that an ancient magma ocean swirled beneath the Earth’s surface and would account for the discrepancy.

Over its 4.5 billion history, Earth’s layers of molten rock evolved and crystallized, cooling from a magma ocean (yellow) to the mantle (grey) around the planet’s core (orange). (Credit: Image courtesy of University of British Columbia)

“As the thick melted rock cooled and crystallized, the solids that resulted had a different composition than the melt,” explains Hernlund, a post-doctoral fellow at UBC Earth and Ocean Sciences.

“The melt held onto some of the elements. This would be where the missing elements of chondrite are stored.”

He says this layer of molten rock would have been around 1,000 km thick and 2,900 km beneath the surface.”

Hernlund’s study explores the melting and crystallization processes that have controlled the composition of the Earth’s interior over geological time. Co-authors are Stéphane Labrosse, Ecole Normale Superieure de Lyon and Nicolas Coltice, Université de Lyon.

The centre of Earth is a fiery core of melted heavy metals, mostly iron. This represents 30 per cent while the remaining 70 per cent is the outer mantle of solid rock.

Traditional views hold that a shallow ocean of melted rock (magma) existed 1,000 km below the Earth’s surface, but it was short lived and gone by 10 million years after the formation of Earth.

In contrast, Hernlund’s evolutionary model predicts that during Earth’s hotter past shortly after its formation 4.5 billion years ago, at least one-third of the mantle closest to the core was also melted.

The partially molten patches now observed at the base of the Earth’s mantle could be the remnants of such a deep magma ocean, says Hernlund.

Correspondence to: S. Labrosse1 Correspondence and requests for materials should be addressed to S.L. (Email: stephane.labrosse@ens-lyon.fr).

The distribution of geochemical species in the Earth’s interior is largely controlled by fractional melting and crystallization processes that are intimately linked to the thermal state and evolution of the mantle. The existence of patches of dense partial melt at the base of the Earth’s mantle, together with estimates of melting temperatures for deep mantle phases and the amount of cooling of the underlying core required to maintain a geodynamo throughout much of the Earth’s history, suggest that more extensive deep melting occurred in the past. Here we show that a stable layer of dense melt formed at the base of the mantle early in the Earth’s history would have undergone slow fractional crystallization, and would be an ideal candidate for an unsampled geochemical reservoir hosting a variety of incompatible species (most notably the missing budget of heat-producing elements) for an initial basal magma ocean thickness of about 1,000 km. Differences in 142Nd/144Nd ratios between chondrites and terrestrial rocks can be explained by fractional crystallization with a decay timescale of the order of 1 Gyr. These combined constraints yield thermal evolution models in which radiogenic heat production and latent heat exchange prevent early cooling of the core and possibly delay the onset of the geodynamo to 3.4–4 Gyr ago.